Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $28.36   4.219%   $29.5565   $29.56   $29.55   $29.60  
  Purchase #2 $106.51   13.682%   $121.0827   $121.08   $121.10   $121.10  
  Purchase #3 $198.37   11.500%   $221.1826   $221.18   $221.20   $221.20  
  Purchase #4 $72.41   5.526%   $76.4114   $76.41   $76.40   $76.40  
  Purchase #5 $118.89   8.324%   $128.7864   $128.79   $128.80   $128.80  
  Purchase #6 $109.83   10.246%   $121.0832   $121.08   $121.10   $121.10  
  Purchase #7 $99.76   12.289%   $112.0195   $112.02   $112.00   $112.00  
  7 purchase totals = $734.13       $810.1223   $810.12   $810.15   $810.20